Fix initial window size again; also always set Qt::WA_NativeWindow

The Qt::WA_NativeWindow attribute is need so that windowHandle() doesn't
return 0, since windowHandle() is used in two places now in the code, just
always set it.

When rememberWindowSize() is false, override the Window-Maximized config
key, which maximizes the window initially.

To test:
- unset "remember window size"
- close the window and make sure the config has been saved to konsolerc
- open konsole, keep the window unmaximized, close, open -> size is restored
- open konsole, maximize the window, close, open, the window is maximized
  even when rememberWindowSize() is false